As integrated circuits (ICs) have become progressively more microminiaturized to provide higher computing speeds, the low dielectric constant polymers used in the manufacturing of the ICs have proven to be inadequate in several ways. Specifically, they have not had sufficient thermal stability, generate toxic byproducts, are inefficient to manufacture, and the dielectric constants are too high.
During the past few years, several types of precursors have been used to manufacture polymers with low dielectric constants for use in manufacture of integrated circuits (IC). Transport Polymerization (TP) and Chemical Vapor Deposition (CVD) methods have been used to deposit low dielectric materials. The starting materials, precursors and end products fall into three groups, based on their chemical compositions. The following examples of these types of precursors and products are taken from Proceedings of the Third International Dielectrics for Ultra Large Scale Integration Multilevel Interconnect Conference (DUMIC), Feb. 10-11 (1997).
I. Modification of SiO.sub.2 by Carbon (C) and Fluorine (F)
The first method described is the modification of SiO.sub.2 by adding carbon and/or fluorine atoms. McClatchie et al., Proc. 3d Int. DUMIC Conference, 34-40 (1997) used methyl silane (CH.sub.3 --SiH.sub.3) as a carbon source, and when reacted with SiH.sub.4 and the oxidant H.sub.2 O.sub.2 and deposited using a thermal CVD process, the dielectric constant (K) of the resulting polymer was 3.0. However, this K is too high to be suitable for the efficient miniaturization of integrated circuits.
Sugahara et al., Proc. 3d Int. DUMIC Conference, 19-25 (1997) deposited the aromatic precursor, C.sub.6 H.sub.5 --Si--(OCH.sub.3).sub.3 on SiO.sub.2 using a plasma enhanced (PE) CVD process that produced a thin film with a dielectric constant K of 3.1. The resulting polymer had only a fair thermal stability (0.9% weight loss at 450.degree. C. in 30 minutes under nitrogen). However, the 30 min heating period is shorter than the time needed to manufacture complex integrated circuits. Multiple deposition steps, annealing, and metalizing steps significantly increase the time during which a wafer is exposed to high temperatures. Thus, this dielectric material is unsuitable for manufacture of multilevel integrated circuits.
Shimogaki et al., Proc. 3d Int. DUMIC Conference, 189-196 (1997) modified SiO.sub.2 using CF.sub.4 and SiH.sub.4 with NO.sub.2 as oxidant in a PECVD process. The process resulted in a polymer with a dielectric constant of 2.6, which is lower than that of SiO.sub.2.
However, one would expect low thermal stability due to low bonding energy of sp.sup.3 C-F and Sp.sup.3 C-Si bonds (BE=110 and 72 kcal/mol., respectively) in the film. The low thermal stability would result in films which could not withstand the long periods at high temperatures necessary for integrated circuit manufacture.
II. Amorphous-Carbon (.alpha.C)- and Fluorinated Amorphous Carbon (F-.alpha.C)-Containing Low Dielectric Materials
The second approach described involves the manufacture of .alpha.-carbon and .alpha.-fluorinated carbon films. Robles et al., Proc. 3d lnt. DUMIC Conference, 26-33 (1997) used various combinations of carbon sources including methane, octafluorocyclobutane and acetylene with fluorine sources including C.sub.2 F.sub.6 and nitrogen trifluoride (NF.sub.3) to deposit thin films using a high density plasma (HDP) CVD process.
The fluorinated amorphous carbon products had dielectric constants as low as 2.2 but had very poor thermal stability. These materials shrank as much as 45% after annealing at 350.degree. C. for 30 minutes in nitrogen.
One theory which could account for the low thermal stability of the fluorinated amorphous carbon products is the presence of large numbers of sp.sup.3 C-F and sp.sup.3 C-sp.sup.3 C bonds in the polymers. These bonds have a bonding energy of 92 kcal/mol. Thus, the films can not withstand the long periods of high temperatures necessary for IC manufacture.
III. Fluorinated Polymers
The third approach described uses fluorinated polymers. Kudo et al., Proc. 3d Int. DUMIC Conference, 85-92 (1997) disclosed polymers made from C.sub.4 F.sub.8 and H.sub.2 H.sub.2 with a dielectric constant of 2.4. The polymers had a Tg of 450.degree. C. (Kudo et al., Advanced Metalization and Interconnect Systems for ULSI Applications; Japan Session, 71-75 (1996)).
However, despite its low leakage current due to presence of sp.sup.3 C-F bonds, a low thermal stability can be expected due to presence of sp.sup.3 C-F and sp.sup.3 C-sp.sup.3 C bonds in the films. Thus, like the F-.alpha.C-containing polymers discussed above, these fluorinated polymers are unable to withstand the prolonged high temperatures necessary for IC manufacture.
LaBelle et al, Proc. 3d Int. DUMIC Conference, 98-105 (1997) made CF.sub.3 --CF(O)--CF.sub.2 polymers using a pulsed plasma CVD process, which resulted in a polymer film with a dielectric constant of 1.95. However, in spite of the low K, these polymer films would be expected to have low thermal stability due to presence of sp.sup.3 C-sp.sup.3 C and sp.sup.3 C-O bonds in these films which have bonding energies of 85 kcal/mol.
Therefore, none of the previously described low dielectric materials have suitably low K and high thermal stability necessary for IC manufacturing.
Wary et al, (Semiconductor International, June 1996, 211-216) used the precursor, (.alpha.,.alpha.,.alpha..sup.1, .alpha..sup.1)tetrafluoro-di-p-xylylene) or {--CF.sub.2 --C.sub.6 H.sub.4 --CF.sub.2 --}.sub.2 Parylene AF-4.TM., which contains a non-fluorinated aromatic moiety, and a thermal CVD process to manufacture Parylene AF-4.TM. which has the structural formula: {--CF.sub.2 --C.sub.6 H.sub.4 --CF.sub.2 --}.sub.n. Films made from Parylene AF-4.TM. have a dielectric constant of 2.28 and have increased thermal stability compared to the above-mentioned dielectric materials. Under nitrogen atmosphere, a polymer made of Parylene AF-4.TM. lost only 0.8 % of its weight over 3 hours at 450.degree. C.
However, in spite of the advantages of conventional poly(para-xylylenes), there are disadvantages of the known methods of their manufacture. First, the manufacture of their precursors is inefficient because the chemical reactions have low yields, and the process is expensive and produces toxic byproducts. Further, it is difficult to eliminate redimerization of the reactive intermediates. When deposited along with polymers, these dimers decrease the thermal stability and mechanical strength of the film.
Thus, the prior art contains no examples of dielectric material precursors for semiconductor manufacture which have desired properties of low dielectric constant, high thermal stability, and low cost.